1. Field of the Invention
The present invention relates to GaAs wafers and particularly to a method and system for automated measurement of whole-wafer etch pit density in GaAs wafers.
2. Description of the Prior Art
One of the prime factors affecting large-scale device development in GaAs is the etch pit or dislocation density. Although there is some disagreement on whether or not a single dislocation near a device such as a field-effect transistor (FET) can significantly affect the electrical behavior of that device, there seems to be general agreement that a large number of dislocations within the region surrounding the device will indeed affect its electrical properties. A good way to study such effects is to compare the variations in dislocation density over a large area, such as a 2-inch or 3-inch wafer, with the corresponding variations in some critical device parameter, such as the threshold voltage. Then standard statistical techniques can be used to determine the degree of correlation. Modern autoprobe techniques can be used to map the particular device parameter, even up to many thousands of devices. However, there is no such convenient technique to map the dislocation density. The standard method is to place the wafer in hot KOH, forming small etch pits where dislocations intersect the surface, and then to count the pits within a given area about the point of reference. This procedure is very tedious, so that seldom are more than lo positions analyzed on a given wafer, making good correlation with device properties nearly impossible.
U.S. Pat. No. 4,637,726 discloses a system for mapping GaAs and other semiconductor wafers for areas of nonuniformity. The wafer is positioned in a magnetic field and illuminated by a light source at a plurality of points on the surface thereof. A computer initializes the light level and the electronic gain of each detector preamp associated with a fiber optic link from an analyzer. The magnetic field direction is then reversed by computer command, causing a localized change in intensity cf light passing through the wafer due to Faraday Rotation. The resulting change in detector output together with location and wavelength data can be used to compute a map of the wafer.
U.S. Pat. No. 4,740,708 discloses a system and procedure for the inspection of the surface of a semiconductor wafer which ascertains that particulate contaminants have been adequately cleaned from the surface during the manufacture of integrated electric circuits The wafer is advanced in a first direction and is optically scanned in a second direction, transverse to the first direction, for recording intensities of light reflected normally from the wafer surface as a function of location on the scan line.
However, none of the above references teaches a method or system for obtaining whole-wafer dislocation (etch pit density) maps of a GaAs surface.